I have always wondered why. Das Rheingold and Siegfried have always been my favorites from the Ring Cycle (usually Rheingold, although there was a long stretch when I liked Siegfried even more) but never Die Walküre or Götterdämmerung. Don't get me wrong, I love them but when it comes to personal preferences there is no doubt in my mind that Rheingold and Siegfried are superior.
Is the reason for the popularity of Walküre and Götterdämmerung the element of humanity? Rheingold and Siegfried lack particularly likable characters (in fact I find Siegfried's treatment of Mime borderline unbearable) so for all the complex and understandable portrayal of characterizations and motives, all that violence without the soothing element may seem too rough. But that's actually what I don't get: people usually praise Siegmund and nowadays I actually like him - in act 1. In act 2 he becomes unbearably clingy, to the point of declaring that no one else than he (her brother (!) has a right to touch Sieglinde, to the point that he is prepared to kill Sieglinde in her sleep. Yes, very humane indeed. And the popularity of Götterdämmerung can be explained with the fact that it is the only opera of the cycle in which humans in general are really dominant as a race. No more mythological characters (not much, that is).
Or perhaps people like the music in those two operas more? That kind of baffles me, too. Siegfried is pretty murky in the beginning, true, but that makes the slowly emerging light all the more spectacular. And maybe murky is a wrong word, except for the prelude of act 1 which is pretty gloomy, even if in a good way. Much of act 1 as well as act 2 however has warm sound, in low string instruments, yes, but it feels pleasant to my ears. And the waldweben of act 2 is absolutely sparkling with light, much like Act 3 and much of Das Rheingold.
And the action element - IMO Rheingold and Siegfried are among the most action-oriented Wagner operas (which isn't necessarily saying much, though) whereas Walküre and Götterdämmerung are more static. So what is it that the audience sees in Walküre and Götterdämmerung what Rheingold and Siegfried (supposedly) lack?
Any ideas?